Soldering of through-hole components and power electronics: what if selective soldering was the solution?

Power electronics, i.e. electronics applied to the management of high currents, is increasingly present in the electronics industry. This is true not only for the automotive industry, with the electrification of the vehicle fleet, but also for the electrification of the aeronautical, military, lightning and medical industries.

These uses in electronics imply constraints that must be taken into account to guarantee quality and reliability, both in terms of design and board assembly.

Power electronics often differ from so-called traditional electronics in that they are often characterized by :

A difference in printed circuit board (PCB) design, with possibly :

• thicker dielectric substrates for better electrical insulation
• thicker copper - (copper thicknesses of 70µm - 120 µm are frequently used for power applications, compared with 18 to 35 µm for more traditional applications) .
• This greater copper thickness will allow higher currents to flow and avoid reaching breakdown voltage or track delamination.
• track designs and greater electrical insulation distances between tracks, again to allow higher currents to flow and avoid arcing
• specific substrates such as dielectric-coated aluminum for applications requiring high heat dissipation

and a difference in the components used, whose design is adapted specifically for power, often with :

• component housings with larger dimensions than their equivalents used for so-called traditional electronics
• metal component housings
• heat sinks directly integrated on components or added close to them
• more through-hole components than SMD components and therefore mixed boards .

All these characteristics have a major impact on the thermal profiles that need to be implemented to guarantee the quality and reliability that manufacturers are always looking for in their products.

Generally speaking, these characteristics will tend to increase the inertia and thermal dissipation, and therefore the energy absorption, of the board's component parts, which is an essential point to take into account in PCB design and the parameters to be implemented during assembly. For further information on PCB design, please refer to standards IPC-2221 / IPC-2222 / IPC-2152 / IPC-J-STD-001H / IPC-A-610H / IPC-J-STD001HA and IPC-A-610HA.

As far as assembly is concerned, many power electronics components are through-hole components and require connectors. In power electronics, many boards are a mixture of surface-mounted components (SMD) and through-hole components. In addition to the reflow oven soldering process generally used for SMDs, it is therefore necessary to use another method for soldering through-hole components. There are 4 techniques available to operators:

The 4 soldering techniques

• Intrusive soldering

  using solder cream or solder preforms (preforms are inserted between the PCB and the component). The board is then fully soldered as it passes through the reflow oven (SMD & through-hole are soldered at the same time). This technique is generally reserved for a few specific components on mass production lines, due to the time required for design and qualification, as well as the higher unit cost than other techniques. This technique has the advantage of being reliable and guaranteeing good repeatability once the process has been qualified. Inerting, which is desirable for power electronics, will be provided by a reflow oven offering this feature, such as a vapor-phase reflow oven or an oven under nitrogen inerting.

• Soldering with a manual soldering iron:

  through-hole components are soldered using an off-line soldering iron (soldering station) (commonly called a soldering iron). This technique is still used in Europe, but is generally reserved for prototyping, small-scale production or repair. The ease with which it can be implemented is often the reason for choosing this technique, which is however more complicated to master in terms of reproducibility, and does not allow for high production rates. In addition, it is difficult to offer inerting with this method. It should be noted that irons generally have to be adapted to this kind of application, in particular with power ratings and breakdowns adapted to greater thermal dissipation and inertia. Induction iron technologies are the answer to this problem. The METCAL range (MX & CV series) is perfectly suited to these applications.

• Semi-automatic or automatic soldering with soldering robots:

  The advantages and disadvantages of this method are much the same as with manual soldering. It should be noted, however, that inerting can be implemented more easily. However, it is more complicated to increase throughputs, especially if there are a large number of components to be soldered on the board (unfavorable "takt time" coefficient).

• Wave soldering:

  traditional wave soldering is ideal for boards with a complete side to be soldered through, and for mass production. Otherwise, there are design and assembly constraints when it comes to installing SMD components on the through-hole side. On the design side, the direction of passage through the wave must be taken into account to ensure correct soldering. On the assembly side, an additional step is required to bond the components before they pass through the wave, so that they don't fall out when they come into contact with the molten alloy. This latter component bonding technique is often reserved for mass production, due to the constraints involved. Another technique is to use a selective soldering frame to mask SMD components that have been reflowed on the solder source side. Waves are fully compatible with the nitrogen inerting process. However, they present a notable difficulty for power electronics applications. Large heat sinks and capacitors will require a longer exposure time in the wave than smaller components to reach soldering temperature; in this case, smaller components will suffer thermal stress, resulting in instantaneous or latent failures.

• Off-line and on-line selective soldering:

  the selective soldering technique enables the wave soldering technique to be used locally. SMD components are soldered in the traditional way, using a reflow oven. After insertion of the through-hole components, the board is fluxed and preheated to activate the flux and allow the board and its components to rise in temperature. A wave of alloy emanating from a nozzle, the diameter of which is defined according to need, forms the wave. The nozzle is surrounded by a flow of nitrogen to ensure inerting, and by moving the board, the nozzle soldering only the components passing through it. For more complex locations, special frames can be configured to protect adjacent SMDs. The advantage of this technique is that it is perfectly suited to prototyping, small-scale production or mass production in line. Versions dedicated to power electronics are available on the market, as is the case with SASINNO.

The main difficulties encountered in power electronics when soldering through-holes are as follows:

• 1. Oxidation

Inerting enables oxygen to be expelled during the soldering process and prevents oxidation. Depending on the inerting process used, the oxygen contained in the air is either eliminated or replaced by a neutral gas. These inerting processes are implemented either by vapour phase brazing, vacuum brazing or brazing in a nitrogen-saturated atmosphere. Inerting greatly reduces oxidation over time, improving the solderability of circuits and components, and hence the reliability of the final product. Extreme oxidation can weaken solder joints, leading to false contacts or even solder joint breakage when high currents are used. Moreover, working in an inert atmosphere greatly reduces the formation of slag in the bath and the clogging of nozzles.

• 2. Dry brazing (fam. Dry soldering)

During soldering, if the component and PCB are not at the same temperature (or if impurities or oxidation are already present before assembly), dry soldering may occur. This defect may be difficult to recognize, as an electrical test may be conclusive. However, over time, oxidation can occur between the solder and the PCB or component leg, causing random failures. The passage of a strong current over a dry solder joint can lead to arcing or a clean breakdown of the soldered joint.

• 3. A cold solder joint (fam. Cold solder)

When the components and board to be soldered are not at the right temperature and/or the thermal inertia of the wave is insufficient, the alloy may set too quickly. The quantity of alloy deposited is then too small to be electrically functional (in a compliant manner) and the braze joint non-uniform. Cold solder joints are easily recognized visually. Electrical tests may be conclusive, but the lack of alloy will not allow strong currents to flow, at least not permanently.

• 4. voids (or micro-cavities)

These cavities can form during the brazing process and trap air or residues such as flux residues. It is essential to work with suitable fluxes and to ensure the absence of impurities and oxidation. Adjusting the thermal profile will also be decisive in preventing their formation. To limit voids, it is essential to control the process with a high degree of repeatability. In reflow soldering, working under vacuum (vapour phase or convection) can significantly reduce voids in soldered joints, but these processes are not feasible for wave soldering. Only X-ray inspection can validate the absence of voids in a brazed joint. The absence of voids is an essential element for power electronics that must not be overlooked. The presence of these cavities will modify the thermal gradient and the current lines. This in turn leads to changes in electrical resistance and, more specifically, to overheating, which can cause soldered joints to break. For further information, please refer to the study of the impact of micro-cavities (voids) in the chip attachments of power electronic modules by Son Ha Tran - Université Paris Saclay (COmUE), 2015. French. NNT: 2015SACLN010ff. tel-01266057

• 5. No alloy rise in plated holes

In order to have compliant, operative and reliable brazed joints, it is essential to have good alloy rise in the brazed joints along the component leg in the thickness of the dielectric substrate used. The soldered joint on the face of the solder source (visible face) can be visually perfectly compliant without the alloy rising along the component legs. This ensures that high currents do not flow, by limiting overheating due to the passage of the current, and thus guaranteeing the absence of arcing and breakage of the brazed joints or, worse still, of the component leg. To avoid this phenomenon, the design itself is important, i.e. defining an appropriate diameter for the component leg, using thermal brakes on holes connected to ground planes, and raising the components so as not to obstruct the holes and hinder the rise of alloy. Secondly, preheating is an essential and crucial element. It is imperative that the alloy is not cooled during the ascent, and therefore not frozen by the temperature of the dielectric or the component. This also calls for a brazing pot (fam. solder pot) with sufficient thermal inertia. The advantage of selective soldering for this latter problem, especially compared with traditional wave soldering, is that a different, and therefore optimal, setting can be chosen for each component (e.g., speed of passage through the wave). Selective waves with preheating devices that are more efficient than those used for traditional electronics are available on the market to guarantee perfect reliability and repeatability in all the above-mentioned difficulties. This is the case for the SASINNO selective soldering range, which has been recognized for the quality and innovation of its equipment for several years now, as demonstrated by the Global Technology Award in the "Soldering" category presented in 2020.

Sasinno offers selective soldering equipment perfectly suited to power electronics. Mixed preheating solutions (infrared & convection heating) are used on in-line equipment and for the most demanding applications. The brand is able to produce specific designs to customer specifications. For example, SASINNO has developed the MAStamp-151 in-line selective soldering system to meet a demanding customer application for soldering power charger boards for the automotive industry. This highly demanding application, with a board height of 200 mm and an individual weight of 15 kg for mass production, was made possible by the 5 upper and lower preheating zones and a conveyor capable of accepting up to 107 kg of load spread over all 7 zones! (i.e. zone 1: fluxing, zones 2 to 6: top & bottom preheating in mixed technology [infrared & convection], zone 7: selective soldering).

Special requirements, such as dual-line conveyors or additional soldering pots, are available on request. The entire Sasinno range is always offered with nitrogen inerting as standard.

Overview of machine zones